During the drilling process for the exploration and production of oil and gas reserves the downhole conditions require a wide range of wellbore fluids for both the drilling and completion operations. The drilling process utilises complex drilling mud formulations which may be based either on water or oils (predominantly on mineral or synthetic oils). Completion operations also require the use of a range of fluids, from fresh water to highly saturated brines.
Oil-based muds are superior to water-based muds and are used particularly where drilling is difficult. For example, oil-based muds are used exclusively in North Sea oil exploration and production operations, as a consequence of the practice of directional and horizontal drilling. In other geographical locations where environmental legislation permits its use the use of diesel is often the preferred base fluid due to its preferential rheological properties and economic advantages. Particularly in the North Sea more expensive “synthetic” or “low tox(icity)” base oils or vegetable oils may be used as alternatives based, for example, on various esters, alpha olefins or plant oils.
Oil-based muds are oil continuous and consist of an oil, which may be of mineral or synthetic origin, plus detergent plus variable amounts of colloidal clay and weighting agent (e.g. bentonite). Other additives such as rheology control agents, polymers, biocides and corrosion inhibitor chemicals are also employed. During drilling mud is pumped continuously to the drill head. The oil itself lubricates and cools the drill bit, whilst the colloidal clays act as wall-building agents, acting to stabilise the wellbore. Oil and synthetic based fluids are also used to overcome borehole stability problems which may be caused by the hydration and swelling of clays in drilled shale zones when in contact with water. These demands require a fluid possessing a density sufficient to withstand hydrostatic pressures as the well is drilled deeper whilst minimizing the swelling of the drilled shale zones. Mud returning to the surface acts as a fluid transport system carrying the rock cuttings back to the surface, which vary extensively in their geologies depending on the character of the rock formation being drilled.
Water based muds are sufficient in simple drilling projects and for lower temperature wells in moderate depths. These systems require the use of wetting agents to disperse drilled solids and weighting materials. Surfactants are also often employed. These serve to improve fluid tolerance to ion contamination and to increase inhibitive properties of the fluid in drilled shales. These mud systems are also used to alter and control and/or enhance temperature stability, corrosiveness and foaming tendencies of the drilling fluid.
Muds are predominantly invert emulsion systems i.e. oil continuous systems containing a saturated internal water phase stabilised by emulsifiers and oil wetting agents which also reduce the viscosity of the mud. Due to cost, safety, and performance capabilities the oil based muds are predominantly used in preference to water based muds.
As a result of these drilling operations a “cake” of oily drilling mud and produced cuttings is left behind on all surfaces downhole and the surfaces are left in an oil wet, rather than a water wet, condition. These surfaces not only include the wellbore casing and other such equipment but also the surface of the formation itself. This situation poses a number of difficulties for completion and post completion workover operations designed for reworking a well and thence stimulating and optimising production. For example, cementing and gravel packing procedures require a water wet surface in order to allow efficient cement bonding. The removal of oily residues and solid particulate material is also required to allow the unhampered economic running of further engineering downhole e.g. screening exercises. Once the completion process is finalised the formation and screens in place must be permeable in order to allow either oil production or the processes of Enhanced Oil Recovery (EOR) by the pumping of produced water into the formation for example.
In a similar fashion tanks that have been holding oily materials and oily wastes from the above processes are required to be cleaned thoroughly before drilling mud changeovers and before the storage and circulation of brines in the completion process in order to avoid cross contamination events. In addition the oily wastes themselves may require remediation treatment.
The wellbore cleaning methods employed use a whole variety of wellbore cleanup and completion fluids to displace drilling fluids once the producing zones have been drilled and isolated in the wellbore. The result is an extremely complicated process using multiple spacers of many different types. Large volumes of organic based fluids are often required as pre-treatments and considerable amounts of waste (typically circa 1000-5000 bbls) are produced in the process. There are also often problems and complications where different spacers and fluids mix at their interfaces with the same result of producing complex mixtures of waste for treatment and/or disposal.
Once the production process itself begins various quantities of sand and water may be produced with the crude oil and this mixture requires separation prior to the crude oil being pumped through pipelines or transported to suitable refinery or storage facilities prior to processing. A range of treatment and separation processes may be applied in order to effect an efficient separation of sand, water and oil and often simple gravity separation or centrifugation methods are used.
In sand separators for example the solids settle out to the bottom of the tank. The water phase acts as a spacer/interface and the oil phase, being least dense, separates to the surface and is pumped away. Over a period of time considerable volumes of crude oil contaminated solids and sludges build up within these tanks and separators and this requires removal and treatment prior to disposal. In some cases the separation and/or storage tanks may be extremely large indeed, and offshore such tanks can make up a considerable proportion of the fabric of a rig and can be constructed of either metals or even concrete. In this instance the tanks are not cleaned on a regular basis but there is increasing pressure on the oil and gas industry for remediating these tanks prior to dismantling the structures as a major part of the decommissioning process.
Various centrifugation systems are also often used for enhanced separation and these systems often yield a crude oil contaminated solids phase as a waste bi-product also requiring treatment and disposal.
Currently and historically multipurpose industrial cleaning products are used to remove and/or clean and remediate these oily contaminants. These chemicals are often based on organic solvent solutions and blends of organic solvent products eg. Shellsol (Trade Mark) (manufactured by Shell Chemicals) and Super Pickle (Trade Mark) (manufactured by WellFlow Technologies) are often utilised. Sometimes these solvents have relatively low flash points and, for example, may comprise of (synthetic) base fluids, diesel, terpenes, xylene, benzene or toluene or mixtures thereof which simply act as an organic solvent to dissolve the organic materials and free the residues contained on the contaminated surfaces.
Often additives are included within the organic solvent to enhance their cleaning performance and to contribute other properties e.g. U.S. Pat. No. 5,333,698 is based on a non-toxic white mineral oil with additives such as wetting agents, viscosifiers, weighting agents, particulate agents etc. Other solvents may also be used. For example, U.S. Pat. No. 5,773,390 outlines the use of terpene alcohols as solvent additives in cleaning systems but these systems are not claimed as being microemulsions or microemulsion forming systems and the surfactants used are not microemulsion forming surface active agents. More commonly used as co-solvents in other examples in this field are xylene, toluene, and benzene and mixtures thereof.
Some previous patents outline the addition of salts dissolved in organic solvents e.g. U.S. Pat. No. 4,514,310 which outlines the use of N-methyl-2-pyrrolidone as a non-aqueous solvent to dissolve a density increasing salt eg. CaBr2 or KSCN and to enable miscibility with water. Again other additives such as viscosity increasing polymers may be incorporated. U.S. Pat. No. 5,556,832 is similar using eg. acetates in dissolving zinc halides and alkaline earth metal halides. Also PCT Patent Specification No. WO/01/77252 discloses the use of a non-aqueous wellbore fluid with formulations of specific ionic liquids in order to enable and enhance electrical conductivity allowing electrical telemetry operations to be carried out allowing, for example, data logging procedures to be carried out remotely during the process.
However, the use of these non-aqueous fluids and the techniques described above may not render the oil contaminated surface clean enough and sufficiently water wet and additional aqueous surfactant washing procedures, pills and spacers may be required following the use of such an organic washing pill.
For these reasons aqueous systems of chemicals are utilised in such well treatment and oil removal/oily waste treatment processes. For example, British Patent Specification No. 2367315 outlines the use of polyol, polysaccharide, weighting agent, and water to form a silica free aqueous solution.
Sequestering agents such as sodium gluconate, EDTA and NTA are often incorporated into these formulations to complex with metal cations which may be present in the contaminating material and which often interfere with the efficiency of the surfactant formulations used.
U.S. Pat. No. 6,140,277 outlines the use of chelating agents N-cis-13-docosenoic-N,N-bis(2-hydroxymethyl)-N-methyl ammonium chloride, HEDP, ATMP, TTPMP, EDTA, CDTA, DPTA, and NTA, and enzyme systems eg. alpha- and beta-amylase in a viscoelastic surfactant (VES) matrix.
Alternatively, US Patent Specification No. 2001/047868 discloses the use of acids and cationic salts in aqueous systems. Such solutions may also comprise other enzymes and oxidisers. Acid treatments and combinations of oxidisers are also disclosed in U.S. Pat. No. 4,934,457. In this example both aqueous hydrogen peroxide, naphtha and hydrochloric acid are used.
Surfactants are also often used in the completion process to form isolating “pills” which are pumped between the drilling and completion fluids. The function of these surfactant formulations is to minimise the contamination of the completion fluid by the drilling mud that it will displace. Surfactants are also used in the completion fluid itself to reduce corrosion and to increase its water wetting properties in the producing zones.
More commonly used surfactant based formulations will comprise chemicals which are usually complex emulsion forming formulations of surfactants. These blends therefore tend to be a mixture of surfactants and other additives such as organic solvents which independently or together are oil or water continuous emulsion forming systems when diluted and dispersed on site in water or brines. U.S. Pat. No. 5,710,111 discloses the use of a non-aqueous (invert oil continuous) emulsion system using unhalogenated organic fluid as a wellbore fluid. U.S. Pat. No. 5,846,913 discloses another oil and water-in-oil (W/O) emulsion where the oil phase comprises a biodegradable alkane which is used as a wellbore fluid.
Polymers may also be used in emulsion systems. PCT Patent Specification No. WO/01/94742 discloses the use of a polymer emulsion for sealing and isolation applications in the wellbore and includes certain water-in-oil (W/O) microemulsion systems stabilised by polymers and cross-linking agents. The use of polymers is also outlined in U.S. Pat. No. 6,279,656 forming a water soluble “shell” with cross-linking agents. In U.S. Pat. No. 5,706,895 a polymer based system is enhanced with a foam fluid for use in workover, completion and in kill fluids. In this instance a non cross-linked polymer is used in conjunction with a surfactant. There are several disadvantages in using emulsion forming surfactant systems outlined below.
Microemulsion systems of certain types are known in the prior art. More commonly these microemulsion systems are designed specifically to have acidic properties or are combined with acids and are used for wellbore cleanup operations and in the processes of acid fracturing of formations and the treatment of subterranean formations in order to stimulate and increase productivity. In these instances it is the acidising properties which are important as one of the main modes of action in carrying out the procedures. Such disclosures are made in U.S. Pat. No. 5,008,026 outlining a water continuous system using glycol ethers as a “mutual solvent” and glycols and alcohols as co-solvent. Similarly US Patent Application Publication No. 2002/0132740 outlines the use of acid based microemulsions for a range of oil industry applications—the formulations outlined therein containing up to 60% wt acid. U.S. Pat. No. 5,034,140 outlines a similar system but this is an oil continuous acid internal system using a hydrocarbon carrier fluid for addition to an acid treatment fluid.
Because such products are highly acidic, aggressive, and highly reactive or contain other strong oxidising or reducing agents their use can entail some very deleterious risks and hazards to both personnel and equipment. These systems are, by their very nature, extremely corrosive or caustic for example and special procedures need be observed with regards to storage, transport, handling and use. Although Health & Safety should be paramount other protocols need to be considered such that screens and other such engineering equipment is not damaged through their use. In addition when such chemicals are used the potential adverse environmental effects from spillages of these formulations could also have ramifications on environmental compliance and performance. Alternatively, some chemical formulations require the use of highly toxic chemicals as part of their constituents eg. butoxy ethanol (ethylene glycol monobutyl ether). While these chemicals may not pose a significant threat to the marine or aquatic environment they certainly pose a potential health and safety threat being severely toxic and poisonous to personnel handling the chemicals.
Certain other microemulsion chemical cleaning systems have been used in the past, for example in wellbore cleanup, and other such oil and gas industry operations. The disclosures are very similar to the above and once again typically combine surfactant with alcohols, glycols and glycol ethers which are predominant as Co-surfactants and co-solvents necessary for the formulations to work.
U.S. Pat. No. 5,762,138 and European Patent Specification No. 0566394 both disclose the use of microemulsion well cleaning formulations. U.S. Pat. No. 5,762,138 incorporates an “anti-sludging” surfactant. The solvents used in this case are glycols and glycol ethers, and an alcohol is used as the co-solvent. This formulation, like those above, is primarily an acidic formulation and may therefore also be used for fracture acidising. European Patent Specification No. 0566394 is very similar but also incorporates a foaming agent additive component. British Patent Specification No. 2347682A uses a simple surfactant and alcohol as co-surfactant combination. These combinations tend to be less effective at removing synthetic base muds from surfaces. Strictly solvent based or surfactant based formulations both suffer from a lack of cleaning efficiency and both produce significant volumes of emulsion wastes which are an expensive industry problem in their own right.
Almost all of the above mentioned products are mirrored in other industries for use as cleaning agents for hard surface cleaning applications such as the inks and printing sectors.
As stated above the primary purpose of oil removal and oily solids cleaning chemicals is to remove oil, solids and associated particulate material which contaminates the surfaces in tanks and systems. As the state of the art formulations are traditionally emulsion forming systems used at low active ingredient concentrations the cleaning process is predominantly achieved by the process of immiscible displacement rather than by solubilisation and other modes of action. This in turn means that some of the surfaces may remain as oil wet surfaces if the wetting properties of the surfactant formulation are found to be lacking.
Emulsion forming surfactant systems have performance disadvantages in that they have relatively high interfacial surface tension properties when compared to microemulsion based systems. As such emulsion systems are less efficient cleaners than microemulsion forming surfactant based systems and larger volumes of washing fluid are required which carries incurred time, volume, transport and cost disadvantages with their use.
There are further substantial practical disadvantages of using emulsion forming surfactants in that the systems naturally produce emulsion type wastes. These emulsions may be stable and therefore very difficult to separate especially when quantities of fine solids are present such as colloidal clays and bentonite as is the case in waste drilling muds. Oily sludge also has a tendency to float or be suspended in water continuous systems and this can significantly interfere with engineering and the operations of recycling the cleaning fluids. Large quantities of oily water and sludge is usually therefore produced as a result of these cleaning processes. In some instances for cleaning out mud pits for example only hot water is often used. This produces an even larger volume of waste requiring treatment and disposal.
Very stringent global environmental legislation is becoming the trend in the industry and this dictates that such (hazardous) wastes must be treated prior to discharge offshore or brought back from remote regions or from offshore to land based waste treatment and disposal facilities. Economic treatment is not always achievable and many operators are beginning to implement zero discharge policies. As such, large volumes of this type of waste require transport to shore and treatment and disposal by waste management companies in this fashion at considerable cost to operators. The extensive logistics required to perform this operation are also expensive and carry considerable associated health and safety risks, especially when the lifting and transportation of skips is required. This latter operation is also severely hampered by adverse weather conditions.
Many of the microemulsion products currently available on the market are temperature sensitive. Wellbore cleanup and other downhole remediating operations in particular are typically carried out at high temperatures downhole of circa 70-150° C. Most microemulsion systems are inherently temperature sensitive and indeed it has been well known for many years by those skilled in the art that phase behaviour can be altered, and phase separation of microemulsion systems can be readily achieved, by simply altering the temperature. This phase separation significantly reduces the effectiveness of the surfactant system employed.
The applicants have found that, if the operational temperature rises to above the cloud points of (microemulsion) surfactant systems, they phase separate and often do not achieve comparable levels of cleaning efficiency as those systems which are not temperature sensitive and which do not phase separate. In the latter case the surfactants remain active in aqueous solution maintaining their chemical cleaning, surface activity, surface tension reduction and detergency capabilities.
In summary a significant problem encountered by the systems of the prior art has been the large volumes of surfactant solution required in order to achieve efficient cleaning. In order to improve efficiency co-surfactants have been employed which in many instances are environmentally unfriendly. The amounts of co-surfactant employed have generally been small compared with the amount of surfactant utilised. However, the large amounts of surfactants employed results in expensive systems. Moreover, the systems of the prior art in general are not suitable for use and/or perform poorly in brine conditions and over large temperature ranges.